1. Field of the Invention
This invention relates to the field of light bulb sockets and holders and in particular relates to lamp sockets of the bayonet type.
2. Background of the Invention
The need for better bayonet socket has become particularly acute with the advent and widespread use of Halogen type light bulbs which draw a considerably heavier current than earlier vacuum-filament incandescent bulbs. Whereas existing bayonet sockets may have been acceptable for use with the older lower current light bulbs, the high current demands of newer lamp bulb technologies call for improved sockets capable of reliably handling heavy currents without arcing and contact deterioration.
Bayonet type sockets for light bulbs are widely used and are characterized by a cylindrical shell which has a bottom at one end and is open at an opposite end. A disc of electrically insulating material in the shell carries one or more electrical contacts. The carrier disc is urged towards the open end by a spring supported on the bottom. The bulb base has laterally projecting detent pegs which make twist-lock engagement within corresponding detent slots in the socket shell for holding the bulb against the spring force, so that the electrical contacts on the carrier disc are urged against corresponding contacts on the bulb base when the bulb is inserted into the socket. The base of the socket shell has a central opening and a mounting for supporting the socket in a light fixture. The mounting may be a threaded stem or a bracket. One or more electrical wires pass through the bottom opening into the shell and are connected to each of the contacts on the insulated carrier for delivering electrical power to the light bulb. In one type of socket, the insulating disc carries a single contact which touches a center contact on the bottom of the bulb base. A return electrical path is provided by the socket shell itself which is metallic and makes contact with the metallic base of the light bulb through the detent pegs. In another type of socket, the insulating carrier supports two contacts which touch two corresponding contacts on the bottom of the bulb base, while the socket shell is not electrically connected to the power circuit and serves only a mechanical function for supporting the bulb.
In conventional bayonet sockets the contacts on the insulating carrier are metallic rivets with an elongated tubular stem mounted in a hole in the carrier disc, and a disc shaped head fixed to the upper end of the stem with a convex contact surface oriented towards the open end of the shell. The hollow stem of each contact element has a lower open end which receives the bare end of an insulated wire. The bare wire is inserted into the open end of the stem, and the stem is squeezed flat to clamp the wire inside the stem, thereby making both a mechanical and an electrical connection. At the same time, the flattened stem can no longer be pulled out of its mounting hole. The flattened stem is enlarged in diameter and cannot pass through the hole in the disc, which keeps the stem from moving through its mounting hole. Conventional practice is to flatten substantially the entire length of the stem which extends on the side of the carrier disc opposite to the contact head. This in effect, fixes the contact element in place on the insulated carrier disc, and keeps it from moving up and down through its mounting hole.
Light bulbs sockets of this type are frequently used with Halogen lamps, and often in outdoor lighting installations. Exposure to humidity and other environmental influences, whether indoors or out, tends to oxidize the contact surfaces of the socket and bulb. Corrosion gradually increases the contact resistance and has been found to eventually induce electrical arcing between the socket contacts and the bulb base contacts. The arcing in turn accelerates deterioration of the contact surfaces and may cause pitting until a marginal electrical connection exists which is wasteful of electrical power and reduces the quality of light produced by the bulb. In severe cases, the electrical contact may be interrupted altogether, causing the light fixture to fail.
In existing bayonet sockets a single, relatively large coil spring is compressed between the insulating carrier disc and the bottom of the socket shell. The purpose of the spring is to urge the contact or contacts on the carrier disc against the corresponding contacts on the lamp base. In practice, however it may happen that the force of the main biasing spring in the socket is largely absorbed by only one of the contacts which stops the carrier disc from adjusting as necessary to bring the other contact on the disc also firmly against the corresponding contact on the bulb base. It is easy for the carrier disc to become slightly tilted so that only one of the contact elements on the disc makes good contact with the bulb base. Also, considerable friction exists between the carrier disc and the socket shell, which can lock the disc in place, particularly after a period of use where the spring is continuously compressed at a particular position by the bulb base. This setting of the disc may keep it from adjusting to compensate for corrosion or arcing damage to the contact surfaces.
Deterioration of the sliding surfaces between the disc and the socket shell, or oxidation of the spring surfaces have a similar freezing effect on response of the carrier disc to changes in the contact surfaces. Poor electrical contact leads to electrical arcing and eventual burnout of the contact surfaces. This problem is common and a solution is needed to increase the reliability and service life of bayonet sockets for light bulbs, particularly when used under adverse environmental conditions.